Recently, a thin film transistor using an In—Ga—Zn—O-based (hereinafter referred to as IGZO) oxide semiconductor thin film in an active layer (channel layer) has been actively researched and developed. The oxide semiconductor thin film can be formed at a low temperature, exhibits higher mobility than that of amorphous silicon, and is transparent to visible light, and therefore, a flexible thin film transistor can be formed on a substrate such as a plastic sheet or a film.
Here, a comparison of various transistor characteristics such as field-effect mobility, process temperature and the like is shown in Table 1.
TABLE 1LTPS low-temperatureOrganic-Oxide-polysilicona-Si:HμC-Si:HTFTTFTMobility100<12-3<1-53-50StabilityΔVTH<11001-2301-2 UniformityΔ◯◯?Δ◯?Film-450300300RT-100RT-350formationtemperature
As shown in Table 1, although a thin film transistor in which an active layer is made from polysilicon is capable of having a mobility of approximately 100 cm2/Vs, since the process temperature is 450° C. or more and thus is very high, the thin film transistor can be formed only on a highly heat-resistant substrate and is not suitable for reduction in cost, increase in area, and flexibilization. Further, since a thin film transistor in which an active layer is made from amorphous silicon can be formed at a relatively low temperature of approximately 300° C., the selectivity of a substrate is greater than that of polysilicon; however, only a mobility of approximately at most 1 cm2/Vs is obtained, and thus, the thin film transistor is unsuitable for high-definition display application. In contrast, in terms of low temperature film formation, since a thin film transistor in which an active layer is made from an organic material can be formed at a temperature of 100° C. or less, the thin film transistor is expected to be applied to flexible display applications using a plastic film substrate having a low heat resistance; however, the thin film transistor can obtain only a mobility equivalent to that of amorphous silicon.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2010-21555 discloses a thin film transistor in which a high mobility layer containing an oxide of IZO, ITO, GZO, or AZO is disposed as an active layer on a side close to a gate electrode, and an oxide layer containing Zn is disposed on a side far from the gate electrode.
Japanese Patent Application Laid-Open (JP-A) No. 2009-170905 discloses a display substrate including a first semiconductor pattern containing at least amorphous silicon on a gate wiring and a second semiconductor pattern containing at least one of elements selected from Ga, In, Zn, Sn, Co, Ti, and Mg and an oxide element O.
Japanese Patent Application Laid-Open (JP-A) No. 2010-161339 discloses a field-effect type transistor including at least a semiconductor layer and a gate electrode provided facing the semiconductor layer with a gate insulating layer provided therebetween, in which the semiconductor layer includes a first amorphous oxide semiconductor layer containing at least one element selected from Zn or In and a second amorphous oxide semiconductor layer containing at least one element selected from Ge or Si, and at least one element selected from Zn or In.
Further, K. Koike et al., Applied Physics Letters, 87 (2005) 112106 discloses a heterostructure field-effect transistor in which a carrier transit layer is formed into a single quantum well by joining ZnO and ZnMgO having different electron affinities.